1. Field of the Invention
The present invention relates to an inorganic nonaqueous electrolytic solution type cell comprising an oxyhalide which serves as an active material for positive electrode and also as a solvent for an electrolytic solution and a negative electrode made of an alkali metal.
2. Description of the Related Art
An inorganic nonaqueous electrolytic solution type cell, which comprises an oxyhalide which is in the liquid state at room temperature (e.g. thionyl chloride, sulfuryl chloride and phosphoryl chloride) as an active material for positive electrode, an alkali metal (e.g. lithium, sodium and potassium) as a negative electrode, a porous carbon molded article as a positive electrode and in which the oxyhalide serves as a solvent for an electrolytic solution, has a large energy density and works at a lower temperature. However, since the oxyhalide and the alkali metal easily react with water, a completely sealed structure with a hermetic sealing is used. (cf. Japanese Patent Kokai Publication No. 160660/1987).
The cell having the hermetic sealing has high sealing completeness and good storage stability. However, because of such high sealing completeness, when such cell encounters abnormal conditions such as high temperature or charging at high voltage, an internal pressure of the cell increases extraordinarily and a cell container is burst by high pressure, so that a large burst noise is made and contents in the cell are scattered and contaminate an apparatus using the cell.
To prevent the above total breakage of the cell, it is proposed to provide an explosion proof mechanism to the cell so as to prevent explosion of the cell by the increase of internal pressure. For example, as shown in FIGS. 1A and 1B. a thin-walled part 1c is formed on a bottom 1a of a cell container 1. When the internal pressure of the cell is increased to a certain pressure through thermal expansion of the electrolytic solution caused by increase of the temperature, the thin-walled part 1c is broken, whereby the explosion of the cell container 1 is prevented (cf Japanese Patent Kokai Publication No. 86234/1988).
In the above conventional cell having the thin-walled part in the bottom, the cell container functions as a negative electrode terminal and a negative electrode 2 contacts the inner surface of the casing 1. If the cell is abruptly heated by, for example, accidentally dropping the cell in the fire, the alkali metal which constitutes the negative electrode 2 melts before the explosion proof mechanism is actuated, the molten alkali metal passes through a separator 4 made of a nonwoven glass fabric to reach a positive electrode 3. Then, the molten alkali metal and the positive electrode vigorously react with each other, whereby heat is vigorously generated and the internal pressure of the cell abruptly increases to burst the cell container 1.